Strange transformer hum

[Atilla]

Here is the thing, when I power up my gainclone, the transformers tend to have a very slight hum. They're of good quality and there is no mechanical noise from them otherwise. Until I close the case. If the lid is off, there's no sound. If I put the top lid on, there's a slight hum coming from inside.

I'm not too sure, but I think I noticed it for the first time when I was making sure that all panels of the enclosure are electrically connected together and tied to safety earth.

So, what could be happening?

This is how the transformers are mounted:

http://lh6.ggpht.com/_4E9BTPdtA5Y/So...0/dscn2732.jpg

[Enzo]

Make sure the mounting bolts cannot touch the top panel. if they do, the bolt and metal cabinet form a "turn" through the toroid - a shorted turn.

[Atilla]

No, they can't, I've made sure of it. I modified the mounting of the transformers exactly to prevent that. The mounting bolts are shorter in height than the tansformer.

[jechentau]

but sounds like some eddy current indeed somehow, by what you stated about after making contact,

and if you change polarity of one primary?

[Atilla]

I'll try to switch polarity on one of the transformers to see what happens.

The bolts are in electrical contact with the case but I don't see how that can create a problem, if they can't make contact with the lid.

[soundchaser001]

Hi There;

If your lid is made of steel or other magnetic alloy and there is leakage flux from the transformer core in the opprofriate direction it will cause the lid to move lake the diaphram of a speaker.

I saw thais once before on a commercial product.

As an experiment you could try a lid made of aluminum or PCB materia to see if the problem persists.

Good luck
Matt

[Atilla]

Okay, changing the polarity of transformers didn't make any difference.

I do believe that there is a very, very quiet hum with the lid open now, but you almost need to put your ear to the case. With the lid on, you can hear it from 20-30cm away.

The transformers are mounted very tight, on top of their rubber insulation that came along with them, so there's definitely no mechanical vibration. They're dead silent when unmounted as well, so whatever it is, it's related to the steel lids and the mounting bolts. OR, some other part of the circuitry, if that's at all possible.

[robmil]

Was this ever solved???

I have a similar problem with one of my designs. As soon as I bring the lid within a few centimeters of the case, the hum level increases! Without the lid, I get no hum. It's a small integrated phonostage. The smallish toroid is mounted in the same case as the phono board - but at a distance.

So to recap: without the lid on there is no hum.
With the lid within 1 or 2 centimeters of the top the hum starts.

Why?

thanks

Rob

[gfiandy]

The escaping magnetic field from the transformer is attracting and repelling the ferrous metal lid. Causing it to vibrate.

You can sometime screen this out with a plate of steel on the transformer to concentrate the flux or even better a can, but make sure you don't create a loop through the transformer with the bolt or you will great a shorted turn. Or upgrade to a trasnformer with a gauss band which should retain much more of the field in the transformer.

Regards,
Andrew

[Elvee]

The transformers are toroidal, and if they are properly wound, practically no leakage flux can escape.

I would rather suspect the electrodynamic forces caused by Ampère's law, between the loops of cable connecting to the supply PCB: the peak charging current of the capacitors must be rather large, and this may be sufficient.

If the lid touches one of the cables when it is in place, it could act as an acoustical amplifier of the vibrations.

See highlights on the pic.

I would try to tie, or change the configuration of those cables.